There is one more important advice for tennis players from the book Maximum Tennis by Nick Saviano.

Engage in positive self-talk

It is critical to converse with yourself in a positive way when you are on the tennis court. Don’t berate yourself or talk negatively about your game. You cannot afford to battle two opponents, the person on the other side of the net and yourself. One opponent is enough. Engaging in negative self-talk will bring you to defeat quicker than a superior opponent. If you miss an easy shot, the worst thing you can do is to reinforce the mistake by verbalizing, “Oh, what an idiot I am. No way I should have missed that shot!” Instead, say something like, “You’ll get it next time.” Or don’t say anything at all and quickly visualize yourself hitting that shot for a winner. You can even take a practice swing to imprint the positive image in your brain. Along those same lines, try to always be optimistic and think in terms of what you want to happen instead of what you don’t want to happen. For example, assume you are serving for the tennis match. During the changeover, think about wanting to hold serve and what you need to do to be successful, as opposed to thinking to yourself, “I can’t lose serve now” or “Just don’t miss your first serve”. The point is the more you tell yourself what you don’t want to happen, the more likely it is to happen. So, always think and project positively and optimistically and focus on positive affirmations.